Deformation Processing And Evolution Of Microstructure And Texture In Mg-RE Alloys

In recent years,the environmental problem is more and more important due to the damages caused by the rapid development of industry.Human beings eager to find"green material"with low cost and superior property as the foundation for the future.With the development and application of magnesium alloys,they have attracted more and more attention from various industries due to their properties such as lightweight and massive reservation,which can further achieve the goal of energy consumption reduction and environment protection.Among many magnesium alloys,wrought magnesium alloys have been most widely used.However,the applications of the wrought magnesium alloys have been prohibited due to their deficiencies such as difficulty to deform and complication in texture.In this thesis,the effects of the kinds,content and way of combined addition of Rare Earth elements on the properties of deformation,microstructure and mechanical properties of the wrought magnesium alloys have been investigated.Nd,Gd and Y,as the typical light,middle and heavy RE element respectively,were used as soluble alloying elements for the smelting of a series of Mg-based alloys.After semi-continuous casting,the Mg-RE alloys were processed by hot extrusion at 380?and 450?and then by hot rolling at 400?.The microstructure of the Mg-RE alloys was observed by metallographic microscope and scanning electron microscopy?SEM?;the phase and macro-texture of the alloys were analyzed by X-ray diffraction?XRD?;the micro-compositions of the alloys were examined by energy dispersion spectrum?EDS?;the micro-texture of both extruded and rolled alloys was analyzed by electron backscatter diffraction?EBSD?technique combined with pole and inverse pole figures.In addition,the tensile properties of the alloys at different deformation directions were tested.Based on the experiments mentioned above,the influence of RE elements on the microstructure,texture evolution and mechanical properties of the wrought magnesium alloys and the strengthening mechanism of the RE elements was discussed.Main conclusions are as follows:?1?Microstructure of the as-cast Mg-RE alloys:the as-cast alloys show dendritic microstructure consisting?-Mg solid solution and coarse secondary phase compounds,which exhibit semi-continuous network morphology crystallizing at grain boundaries.The XRD patterns of as-cast alloys,indicating that the Mg-RE compounds involve Mg₅Gd Mg₂₄Y₅ and Mg₄₁Nd₅.After solution treatment,the coarse secondary phases are mostly dissolved in the?-Mg matrix and small fraction of them is retained with small dimension.In the following thermo-mechanical processes,the mismatch between the matrix and the second phase particles raises the dislocation density around the second phase particles,which is beneficial to the nucleation of recrystallized grains,i.e.the particle stimulated nucleation mechanism,which has important effect on texture weakening of Mg-RE alloys.?2?Microstructure and texture analyses of the wrought Mg-RE alloys:The microstructures show a fully recrystallized,fine and equiaxed grains in Mg-RE alloys,which means the occurrence of dynamic recrystallization after hot processing such as extrusion and rolling.The processed alloys reveal weakened and non-basal texture,being different from conventional Mg-Al series alloys.By analyzing the effects of different types and contents of RE elements on the texture of wrought magnesium alloys,it is found that the texture weakening effect of light RE elements is stronger than that of heavy RE element with the same content.Moreover,compared with binary RE added alloys,single RE added alloys possess less texture weakening effects.The pole figures of deformed alloys show that the texture in Mg-RE alloy is different from the one of pure Mg due to the RE elements.The orientation of the?0002?plain has small angle deflection towards the extrusion or rolling direction according to the vertical direction,leaving two distinct high intensity peaks.Additionally,the?101^<sub>0?plane is mainly located along the transverse direction?TD?of rolled plates.?3?Orientation distribution function?ODF?analysis of the samples of wrought alloys:Two main RE texture components were observed in both binary and ternary RE elements alloyed and extruded Mg-RE alloys.The primary texture component is{011^<sub>3}<02^<sub>23>and the secondary weak texture component is{^<sub>12^<sub>15}<1^<sub>213>.For the extruded samples,the ternary Mg-RE alloys have obviously lower texture density than those of binary Mg-RE alloys,which is mainly caused by the lower texture density of the secondary texture component.The texture density of the rolled is higher than that of the extruded.They have two types of main RE related textures,the primary texture component{01 1^<sub>3}<0 2^<sub>23>and the secondary weak texture component{01^<sub>14}<01^<sub>12>.The effect of RE elements on the texture weakening of rolled Mg-RE alloys is essentially the same for the extruded ones.Therefore,it can be found that the multi-RE elements addition is effective for the texture weakening.In summary,this thesis can provide advices to improve the deformability and to weaken the formation of texture of wrought Mg by RE alloying.